diff --git a/src/shared/tmux/tmux-utils/layout-runner.test.ts b/src/shared/tmux/tmux-utils/layout-runner.test.ts
new file mode 100644
index 000000000..cec208493
--- /dev/null
+++ b/src/shared/tmux/tmux-utils/layout-runner.test.ts
@@ -0,0 +1,54 @@
+import { beforeEach, describe, expect, it, mock } from "bun:test"
+
+import type { TmuxCommandResult } from "../runner"
+
+const layoutSpecifier = import.meta.resolve("./layout")
+const loggerSpecifier = import.meta.resolve("../../logger")
+const runnerSpecifier = import.meta.resolve("../runner")
+const tmuxPathResolverSpecifier = import.meta.resolve("../../../tools/interactive-bash/tmux-path-resolver")
+
+const runTmuxCommandMock = mock(async (): Promise<TmuxCommandResult> => ({
+	success: true,
+	output: "",
+	stdout: "",
+	stderr: "",
+	exitCode: 0,
+}))
+const getTmuxPathMock = mock(async (): Promise<string | undefined> => "sh")
+const logMock = mock(() => undefined)
+
+async function loadEnforceMainPaneWidth(): Promise<typeof import("./layout").enforceMainPaneWidth> {
+	const module = await import(`${layoutSpecifier}?test=${crypto.randomUUID()}`)
+	return module.enforceMainPaneWidth
+}
+
+function registerModuleMocks(): void {
+	mock.module(loggerSpecifier, () => ({ log: logMock }))
+	mock.module(runnerSpecifier, () => ({ runTmuxCommand: runTmuxCommandMock }))
+	mock.module(tmuxPathResolverSpecifier, () => ({ getTmuxPath: getTmuxPathMock }))
+}
+
+describe("enforceMainPaneWidth runner integration", () => {
+	beforeEach(() => {
+		registerModuleMocks()
+		runTmuxCommandMock.mockClear()
+		getTmuxPathMock.mockClear()
+		logMock.mockClear()
+
+		runTmuxCommandMock.mockResolvedValue({ success: true, output: "", stdout: "", stderr: "", exitCode: 0 })
+		getTmuxPathMock.mockResolvedValue("sh")
+	})
+
+	it("#given pane width inputs #when enforceMainPaneWidth called #then delegates resize-pane to shared runner", async () => {
+		// given
+		const enforceMainPaneWidth = await loadEnforceMainPaneWidth()
+
+		// when
+		await enforceMainPaneWidth("%42", 200, 60)
+
+		// then
+		expect(runTmuxCommandMock.mock.calls).toEqual([
+			[[expect.any(String), ["resize-pane", "-t", "%42", "-x", "119"]]][0],
+		])
+	})
+})
diff --git a/src/shared/tmux/tmux-utils/layout.ts b/src/shared/tmux/tmux-utils/layout.ts
index 259498899..7332a897f 100644
--- a/src/shared/tmux/tmux-utils/layout.ts
+++ b/src/shared/tmux/tmux-utils/layout.ts
@@ -1,4 +1,3 @@
-import { spawn } from "../../bun-spawn-shim"
 import type { TmuxLayout } from "../../../config/schema"
 import { getTmuxPath } from "../../../tools/interactive-bash/tmux-path-resolver"
 
@@ -46,7 +45,12 @@ export async function applyLayout(
 	mainPaneSize: number,
 	deps?: LayoutDeps,
 ): Promise<void> {
-	const spawnCommand: TmuxSpawnCommand = deps?.spawnCommand ?? spawn
+	const spawnCommand: TmuxSpawnCommand = deps?.spawnCommand ?? ((args) => ({
+		exited: (async () => {
+			const { runTmuxCommand } = await import("../runner")
+			return (await runTmuxCommand(args[0] ?? "", args.slice(1))).exitCode
+		})(),
+	}))
 	const layoutProc = spawnCommand([tmux, "select-layout", layout], {
 		stdout: "ignore",
 		stderr: "ignore",
@@ -78,12 +82,9 @@ export async function enforceMainPaneWidth(
 			? { mainPaneSize: mainPaneSizeOrOptions }
 			: mainPaneSizeOrOptions ?? {}
 	const mainWidth = calculateMainPaneWidth(windowWidth, options)
+	const { runTmuxCommand } = await import("../runner")
 
-	const proc = spawn([tmux, "resize-pane", "-t", mainPaneId, "-x", String(mainWidth)], {
-		stdout: "ignore",
-		stderr: "ignore",
-	})
-	await proc.exited
+	await runTmuxCommand(tmux, ["resize-pane", "-t", mainPaneId, "-x", String(mainWidth)])
 
 	log("[enforceMainPaneWidth] main pane resized", {
 		mainPaneId,